There are many different types of bearings available today with very little information on the differences between them. Maybe you’ve asked yourself “which bearing will be best for your application?” Or “how do I choose a bearing?” This bearing selection guide will help you answer those questions.
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First off, you need to know that most bearings with a rolling element fall into two broad groups:
- Ball bearings
- Roller bearings
Within these groups, there are sub-categories of bearings that have unique features or optimized designs to enhance performance.
In this bearing selection guide, we’ll cover the four things you need to know about your application in order to choose the right type of bearing.
Find the Bearing Load & Load Capacity
Bearing loads are generally defined as the reaction force a component places on a bearing when in use.
When choosing the right bearing for your application, first you should find the bearing’s load capacity. The load capacity is the amount of load a bearing can handle and is one of the most important factors when choosing a bearing.
Bearing loads can either be axial (thrust), radial or a combination.
An axial (or thrust) bearing load is when force is parallel to the axis of the shaft.
A radial bearing load is when force is perpendicular to the shaft. Then a combination bearing load is when parallel and perpendicular forces produce an angular force relative to the shaft.
To learn more about axial and radial ball bearings, contact our team of engineers!
How Ball Bearings Distribute Loads
Ball bearings are designed with spherical balls and can distribute loads over a medium-sized surface area. They tend to work better for small-to-medium-sized loads, spreading loads via a single point of contact.
Below is a quick reference for the type of bearing load and the best ball bearing for the job:
- Radial (perpendicular to the shaft) and light loads: Choose radial ball bearings (also known as deep groove ball bearings). Radial bearings are some of the most common types of bearings on the market.
- Axial (thrust) (parallel to the shaft) loads: Choose thrust ball bearings
- Combined, both radial and axial, loads: Choose an angular contact bearing. The balls contact the raceway at an angle which better supports combination loads.
Roller Bearings & Bearing Load
Roller bearings are designed with cylindrical rollers that can distribute loads over a larger surface area than ball bearings. They tend to work better for heavy load applications.
Below is a quick reference for the type of bearing load and the best roller bearing for the job:
- Radial (perpendicular to the shaft) loads: Choose standard cylindrical roller bearings
- Axial (thrust) (parallel to the shaft) loads: Choose cylindrical thrust bearings
- Combined, both radial and axial, loads: Choose a taper roller bearing
Bearing Runout & Rigidity
Bearing runout is the amount a shaft orbits from its geometric center as it rotates. Some applications, like cutting tool spindles, will only allow a small deviation to occur on its rotating components.
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If you are engineering an application like this, then choose a high precision bearing because it will produce smaller system runouts due to the tight tolerances the bearing was manufactured to.
Bearing rigidity is the resistance to the force that causes the shaft to deviate from its axis and plays a key role in minimizing shaft runout. Bearing rigidity comes from the interaction of the rolling element with the raceway. The more the rolling element is pressed into the raceway, causing elastic deformation, the higher the rigidity.
Bearing rigidity is usually categorized by:
- Axial rigidity
- Radial rigidity
The higher the bearing rigidity, the more force needed to move the shaft when in use.
Let’s look at how this works with precision angular contact bearings. These bearings typically come with a manufactured offset between the inner and outer raceway. When the angular contact bearings are installed, the offset is removed which causes the balls to press into the raceway without any outside application force. This is called preloading and the process increases bearing rigidity even before the bearing sees any application forces.
Bearing Lubrication
Knowing your bearing lubrication needs is important for choosing the right bearings and needs to be considered early in an application design. Improper lubrication is one of the most common causes for bearing failure.
Lubrication creates a film of oil between the rolling element and the bearing raceway that helps prevent friction and overheating.
The most common type of lubrication is grease, which consists of an oil with a thickening agent. The thickening agent keeps the oil in place, so it won’t leave the bearing. As the ball (ball bearing) or roller (roller bearing) rolls over the grease, the thickening agent separates leaving just the film of oil between the rolling element and the bearing raceway. After the rolling element passes by, the oil and thickening agent join back together.
For high-speed applications, knowing the speed at which the oil and thickener can separate and rejoin is important. This is called the application or bearing n*dm value.
Before you select a grease, you need to find your applications ndm value. To do this multiply your applications RPMs by the diameter of the center of the balls in the bearing (dm). Compare your ndm value to the grease’s max speed value, located on the datasheet.
If your n*dm value is higher than the grease max speed value on the datasheet, then the grease won’t be able to provide sufficient lubrication and premature failure will occur.
Another lubrication option for high-speed applications are oil mist systems which mix oil with compressed air and then inject it into the bearing raceway at metered intervals. This option is more costly than grease lubrication because it requires an external mixing and metering system and filtered compressed air. However, oil mist systems allow bearings to operate at higher speeds while generating a lower amount of heat than greased bearings.
For lower speed applications an oil bath is common. An oil bath is when a portion of the bearing is submerged in oil. For bearings that will operate in extreme environments, a dry lubricant can be used instead of a petroleum-based lubricant, but the lifespan of the bearing is typically shortened due to the nature of the lubricant’s film breaking down over time.
There are a couple of other factors that need to be considered when selecting a lubricant for your application, see our in-depth article “How to Choose the Correct Ball Bearing Lubricant".
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Hello, I have a Clausing Kondia fv-300 knee mill with Ballscrews on X and Y axis. Prototrak A.G.E. control. I have a question regarding tightening up angular contact bearings. Im having trouble on my Y axis. When i tighten them up the axis gets stiff to turn with hand wheel. my manual says to tighten them up to 50 FT.lbs Im on my third set of bearings and still not much luck. right now i have it tightened up to 50IN. LBS. and that seems to work ok and feel good, but i would think this isnt enough pressure to load the ball screw up right. I am able to get 50Ft. LBS. on the X axis no problem. Im Lost cant figure out why this is happening. I have also tried puting .040 shims between the 2 bearings on the outer races so it will load the inner races against the high shoulder.
just to be more specific, i have the bearings in correctly from all i have read. the lettering on each is facing in to each other. and the thicker inside race is facing outword away from each other. I will try to include a pic from my .
Any help would be greatly appreciated.
Thanks, Scott
Did you buy the machine new? Are the ways worn? Does the end bracket that holds the bearings have dowel pins in it? You may want to leave out the pins and leave the bracket loose or just snug and then crank the saddle as far forward or close to you and then tighten the bracket. What I would do would be to check the gib clearance so it has ." slop side to side tested by putting a mag base on knee and indicator or saddle and pulling and pushing end of table and see a lost motion of ." or . on each side for oil. Then check bearing bracket alignment.
Does the Y feed screw have a lock nut on it? I would tighten it up snug and then crank the saddle into the middle of knee, then put your mag base on knee flat and tenths indicator on saddle and push the saddle in and pull it out and keep watching the lost motion. If your a big guy like me (6'3, 290) you should be able to move it if you small get a friend to help push and pull it. I would slowly tighten the nut until you don't get any lost motion. If you want to check torque then you can so you will know what it is set at. Is there a lube fitting going to the bearings or are they sealed? Give that a shot and see what happens and let us know. Rich
The one picture shows an adjustable split lock nut, so that means it to me it does not have a spacer. If it had a spacer you would lock it tight as the thrust would be built in using the spacer. Have you tried what I said above? I didn't write that for fun, I assumed you tried it before asking more questions. Test the machine with the 50 inch pounds as maybe the manual had a typo.
Do you have a blue print? We are maintenance techs and not mind readers or have a crystal ball. I would say they were in there back to back as in the majority of screw assemblies I have worked on that is the way they are. Back to Back are more for strength or thrust and face to face is usually seen in axial run out used in spindle bearings. The Y axis needs thrust more the accuracy of the run-out of the screw. I looked online and could not find a manual for your machine for free, but I found these helps.
If you are really stumped there is a Practical Machinist member named Matt Critchley who lives in Milwaukee who is an excellent machinist and machine tech (great scraper too), if he isn't busy I bet he would come over and help you. I emailed him and see ask him to read this thread. Also Pm'ed Zahnrad who lives in Milwaukee.
Back to back works with and with out spacers, but if it uses a adjustable split nut spacers usually are not used..
http://www.kaydonbearings.com/downloads/catalog300/Kaydon_300_Mounting.pdf
and
page 488 shows back to back and face to face.
http://www.skf.com/binary/77-/Angular-contact-ball-bearings.pdf
I will bet dollars to doughnuts that you will end up making a precision spacer. Years ago I upgraded the thrust bearings in a Bridgeport V2XT to improve over the factory offerings. I was hesitant after finding similar to what you are experiencing, so went back and ordered actual direct replacement bearing sets. They are never identical. Some adjusting/making of spacers to suit was always necessary in my experience. Once I got past that, I placed the upgraded ones in and made spacers to fit. Never ever had an issue with them and machine ran wonderfully.
I am absolutely BURIED in work right now, with barely time enough to remember to breathe. If you can wait a little bit, or do not figure it out by the time I free up, I'll be happy to stop over and take a look. It will not be quickly, though. Aside from the work load, we have a machine coming for delivery and installation, so I am dealing with everything that brings with it, as well. Clear out, move machines around, run electrical, etc...
Member MCritchley might be a better bet than me. He's a smart cookie.
Did not read all posts , I'm sure bearing concentric marks line up was mentioned.. but perhaps not that contact shoulders needs to be square. Some times checking those shoulders for bugs bruises and squareness. Modern machining accepts turning shoulders and faces in chuckers and the like so a . limit may be accepted. Put a . on one end and off the other end . and one can get much stress on a precision bearing.
So any time a bearing problem occurs or just any time in general it is not a bad to check bearing set-shoulder squareness.
Spacers and/or shims variances would often be used for different speed spindles if simply face to face would not be correct for speed requirements..
Yes old school was turning between centers and line boring insides so the precision was just a given, so bearing in a precision assembly would most often be good for a low speed application but might require a slight step for high speed. .
[gets stiff to turn with hand wheel.] defiantly not set correct. For slow turning face to face should be right so I would look for other problems, bearings just held (clamped together) should make zero end play but not run tight.
Also bearing cleaning important because the stuff in a sealed package that looks like oil is not oil but the rust prevention. It needs cleaning with fresh new solvent, wipe dry with not Air Hose Spinning(NOT), dipped in perhaps new spindle oil, set between two clean towels to drip and dry,then assembled with 1/4 to 1/3 fill (or so ) of grease (if a greased bearing). Yes wash and well dry hands before and during bearing work, work in a clean room not the general shop.
And not a bad idea to check your press size or OD of a shaft, as a press harder than intended can expand the ID on some bearings and so make the intended preload off a little. Often a hand wipe with a fresh harder good quality honing stone will show-up any bang or bumps on a bearing-set OD. yes often just hone the OD error to look good and then check for round with a micrometer good enough.
I have see guys ruin a motor or the bearing with pounding it onto a bugged shaft. My dad taught me to stand a motor shaft on the other end, never pound against the free standing motor. And yes most motor bearing are slip fit, not press so after you hone the key way bugs and the OD bugs the bearing slips on..